1. Field of the Invention
This invention relates to a reflective-type mask used in EUV (Extreme Ultra-Violet) exposure.
2. Description of the Related Art
In recent years, the development of EUV exposure technology has been moving forward to miniaturize device elements further (Jpn. Pat. Appln. KOKAI Publication No. 2004-266300). In EUV exposure, a reflective-type mask is used. A reflective-type mask comprises a region (pattern region) including a pattern corresponding to a pattern to be formed on a wafer and a region (light shielding region) which is provided around the pattern region and whose EUV light reflectivity is low. The light shielding region is a region for suppressing leakage of EUV light from a part where adjacent shots overlap.
The pattern region comprises a reflective film which reflects the EUV light and an absorber which is provided on the reflective film and absorbs the EUV light. This absorber has a pattern corresponding to a pattern to be formed on the wafer. The light shielding region comprises an absorber which absorbs the EUV light. This absorber is provided along the periphery of the pattern region. Since the absorbers in the pattern region and the light shielding region are formed by processing the same film, the absorber in the pattern region and that in the light shielding region are made of the same material and have the same thickness. The absorbers typically have a stacked structure comprising a buffer film and an absorption film which absorbs the EUV light.
The absorbers are required to have a mean reflectivity of less than 0.5%. The reason is that, if the mean reflectivity is not sufficiently low, a pattern cannot be formed on a wafer as designed due to the leakage of EUV light from the part where the adjacent shots overlap. That is, the light shielding region cannot perform its function.
To satisfy a mean reflectivity of less than 0.5% using a Cr film as the buffer film and a titanium nitride (TaN) film as the absorption film, the sum of the thickness of the Cr film and that of the TaN film has to be 70 nm or more.
The EUV light is irradiated to the reflective-type mask at an inclination angle of, for example, about six degrees. Thus, if the absorber is relatively thick (70 nm or more) as described above, the shadowing of EUV light occurs in the absorber in the pattern region.
The degree of shadowing effect differs depending on the pattern. FIGS. 39A and 39B show examples. FIG. 39A shows a line and space pattern (L&S pattern) P1 on a reflective-type mask where a direction parallel to the incident direction D of the EUV light is a longitudinal direction. FIG. 39B shows an L&S pattern P2 on the reflective-type mask where a direction perpendicular to the incident direction D of the EUV light is a longitudinal direction. The shadowing effect is greater on the L&S pattern 2 than on the L&S pattern S1. Accordingly, there is a difference in transfer dimensions between the L&S pattern P1 and L&S pattern P2. Moreover, the pattern center position of an exposure transferred image (light intensity distribution) of the L&S pattern P1 deviated from the center position of the L&S pattern P1.
Although the problem resulting from shadowing can be solved by making the absorber thinner, this will increase the mean reflectivity, making it impossible to solve the EUV light leakage problem.